vibration at 30 and 60mph

I have a vibration at around 30mph and 60mph, it's a lot more noticable when I give gas than when I coast. It's indendent of rpm, I tried it in different gears and get the same vibration. It feels like if the motor mounts or tranny mounts are shot.
I have a 2000 Silverado 2500 4x4, ~100K miles.
Anyone know what it could be?

Thanks.

Have you tried balencing the tires?

Yes, tires are balanced and I also had an alignment done about 3 months ago. I guess a tire might have thrown a wheel weight, would unbalanced tire(s) cause vibration at specific speeds?

I'll keep you posted I have the same very suddle Vibration almost shudder at those speeds. I replaced all 3 U-Joints but, that only seemed to help a little. I think it may be one of my drive shafts out of balance or the Idle bearing. My 99 has 120000 Km's on it and it does it at 50km/hr and it's really noticeable at 100Km/Hr.
It goes in on Thursday for more trouble shooting.

Interesting that you have your vibration/shudder at the same speed as I do. Did you find out anything more?

Only that the bonedheads I took it too, f*cked up my seal on the back of my tranny. They had it for the entire day today and didn't do a damn thing, except to say that yes there is a vibration but, we have no idea what it is. Boy was I mad.
Now I have to find somebody else to fix the seal cause, there is no way in hell I'm going to let these guys touch my truck again.

Update:

Ok so I took my truck in and had the driveshafts balanced by a more reputible company in town and wow what a difference. There is only a slight vibration now at 20Km/hr and that's hit; hardly noticeable. What I did notice though is that my springs are starting to sag a big and that could be contributing to the problem.
Certainly a lot happier with my wagon now, that's for sure. :D :D

I am seeing a lot of problems with vibration. I have a Chevy Avalanche 2002 and have severe vibration between 68 and 75 mph. Everyone thinks the problem is with the tires and alignment. It not. I think it is realated to rotating parts. I am going to try balancing the drive shaft and replacing the sleeve bearings.

AV-Fan

av- let me know how that goes... I too have a vibration between 60-70mph. The wheels have all been rebalanced and alignment is fine. I have a 2000 Silverado 2wd.

How much does something like that cost?

I have a slight vibration on the right side of my truck when i start from a complete stop up to about 5-10 mph. For a wile i thought it was my tail pipe on the right side but now im starting to think it might be the 3rd door.

There is a TSB on this problem from GM,
02-04-17-001A JUL 02 Drivetrain - Launch Shudder/Vibration On Acceleration.
I had the same problem on my truck after about 80,000km and it turned out after replacing all u-joints and steady bearing to be the front drive shaft yoke at the transmission. I now have 172,000km and it has never come back. The dealership tried balancing the driveshaft and finally put a complete driveshaft and steady bearing out of a new truck in before it would quit.